diff --git a/.changeset/add-resource-size-field.md b/.changeset/add-resource-size-field.md
new file mode 100644
index 000000000..bef37cb40
--- /dev/null
+++ b/.changeset/add-resource-size-field.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/core': patch
+---
+
+Add missing `size` field to `ResourceSchema` to match the MCP specification
diff --git a/packages/core/src/types/types.ts b/packages/core/src/types/types.ts
index f9e475340..980b7649a 100644
--- a/packages/core/src/types/types.ts
+++ b/packages/core/src/types/types.ts
@@ -915,6 +915,13 @@ export const ResourceSchema = z.object({
      */
     mimeType: z.optional(z.string()),
 
+    /**
+     * The size of the raw resource content, in bytes (i.e., before base64 encoding or any tokenization), if known.
+     *
+     * This can be used by Hosts to display file sizes and estimate context window usage.
+     */
+    size: z.optional(z.number()),
+
     /**
      * Optional annotations for the client.
      */